High-resolution imaging and metrology requirements in critical dimension (CD) metrology have largely rendered conventional optical image-based CD methods irrelevant for some time. The need to image features smaller than 90 nm, as is now routinely encountered in semiconductor manufacturing, has driven CD scanning electron microscopy (SEM) to the forefront for virtually all in-line or at-line CD metrology. Accurate CD SEM metrology has edge detection and modeling requirements at this scale and is faced with increasing high costs, throughput concerns and is generally not well suited to integrated metrology applications. In addition, there is recent work focused on evaluating electron beam induced line shrinkage or surface damage defects. As a result, there has been a recent shift in strategy to evaluate scatterometry as a manufacturing process control tool. Scatterometry has a high throughput, relatively low tool costs compared to an SEM, is less likely to cause target damage, and has shown excellent sensitivity and repeatability. Some of the drawbacks of scatterometry are the large target size requirements, a similar dependence on modeling, and potentially substantial dependence on underlying layers and optical properties.
The measurement requirements of overlay metrology are somewhat different than those encountered in CD metrology and more conventional optical tools continue to be the tool of choice. For the most part, overlay measurements are essentially pitch measurements between two layers and are well suited to image-based analyses which do not require modeling and an accurate estimation of a physical edge within a profile. To accomplish an overlay measurement with a scatterometry tool, modeling is fundamentally required. The challenges in using SEM for overlay are different than scatterometry as the SEM can make overlay measurements in an image-based mode. The challenges are more a result of imaging buried layers, such as imaging through photoresist, along with the increase cost and lower throughput. There has been some interests recently in evaluating SEM for use in overlay since there is a current push to design overlay targets which are composed of features that have device dimensions. Although the industry continues to use conventional optical tools for overlay metrology, there is a desire to evaluate different methods for use with device-sized overlay targets.
The importance of evaluating overlay with targets fabricated at device dimensions is known. One concern is that larger features used in overlay targets may not correctly reflect actual device overlay since the larger feature dimensions behave differently in the stepper optical systems and in subsequent process steps. In addition, overlay targets in the scribe area have always had some error with respect to overlay values in the active area. This concern has become more pronounced with the increased usage of step and scan procedure tools. The conventional scribe area targets do not capture some essential systematic errors which occur throughout the scanned field of view. Evaluations of small in-situ (in-chip) overlay targets and their potential utility and susceptibility to errors have been made. The measurement required of these small in-chip overlay targets and targets made of device sized features are responsible for much of the current development in high-resolution overlay imaging methods.
In parallel with high-resolution optical overlay development, scatterometry has recently drawn attention to its suitability and flexibility for implementation in integrated metrology applications. Optical tools can generally operate in an ambient environment and are well suited to integrated metrology applications. Although CD SEM tools have very good resolution and sensitivity, they are in general not well suited for use in integrated metrology applications such as built-in stepper track metrology capabilities. Several optical scatterometry tools have now been installed in stepper track systems where they operate in an ambient environment with additional complications introduced by the stepper track such as vibration and size limitations. With the growing importance of integrated metrology and tighter feed forward and feedback control, flexible metrology tools capable of operating in an air ambient with space limitations have become more important.
The present invention provides a method of imaging the structural features of a target or article of manufacture using the zeroeth order of diffracted light. The same method may be applied to higher order diffracted light.